Late entry Kiki Bertens stunned world number one Ashleigh Barty with a comeback three-set victory to shake-up the season-ending WTA Finals in Shenzhen Tuesday.

All four players can still qualify from Red Group after Belinda Bencic's three-set victory over Petra Kvitova in the later match.

Bertens looked in deep trouble at a set and break down, but turned the tide with ruthless returning and aggressive play at the net to overwhelm a deflated Barty, who could have sealed a spot in the semi-finals with victory.

The world No.10 won 3-6, 6-3, 6-4 in two hours and nine minutes in a see-saw match featuring 12 breaks of serve.

"Of course you're always ready to play, but I did not see it coming today," Bertens said of her late call-up. "In the beginning I was struggling. As the match continued, I felt better and better."

Bertens was a late replacement for world No.3 Naomi Osaka, who earlier in the day withdrew from the round-robin tournament due to a shoulder injury.

She appeared nervous under the bright lights dropping her first three service games and Barty, who has locked up the year-end No.1 ranking, pounced to run away with the first set.

Barty then endured her own service woes against an increasingly aggressive Bertens, who showcased a soft touch at the net.

A slumping Barty had her serve broken five times through the second and third sets as Bertens stormed to her first victory over the Australian.

Barty lamented letting slip her early lead. "I wasn't able to execute as cleanly when I was up in the score line," she said.